One Year Clinical Evaluation of a Low Shrinkage Composite Compared with a Packable Composite Resin: A Randomized Clinical Trial.

Objectives
The aim of this study was to evaluate the clinical performance of a packable and a low shrinkage methacrylate-based composite after one year.


Materials and Methods
In this clinical trial, 50 class I or II restorations were placed in 25 patients. Each patient received two restorations. The tested materials were: (I) Filtek P60 + Single Bond 2 and (II) Kalore GC + Single Bond 2. The restorations were evaluated by two independent examiners after one week (baseline), six months and one year according to the modified United States Public Health Service (USPHS) criteria. The evaluated parameters included color match, marginal adaptation, anatomical form, retention, surface texture, postoperative sensitivity, marginal staining and secondary caries. Data were then analyzed using Friedman and conditional (matched) logistic regression tests at P<0.05 level of significance.


Results
P60 and Kalore performed similarly at six months and one year (P>0.05). When each composite resin was evaluated independently at baseline and after one year, no statistically significant differences were found except for marginal adaptation (P60) where four restorations were rated Bravo (clinically acceptable). In 8% of restorations, patients expressed postoperative sensitivity.


Conclusions
Kalore GC and Filtek P60 showed acceptance clinical performance after one year of service.


INTRODUCTION
The improvements in dental adhesives and composite resins, along with increased demand for tooth-colored restorations in addition to a minimally invasive approach for caries treatment, have made composites the primary choice for direct posterior restorations in many countries [1,2]. Recent clinical studies indicated acceptable clinical performance of composite restorations and some even provided 10-to 20year outcomes showing low annual failure rates of approximately 2% [3,4]. However, the polymerization shrinkage seems to be the major limitation of composite resins in posterior restorations [5].
This volumetric shrinkage ranges from 2 to 5% which may cause debonding at the toothcomposite interface and subsequently lead to postoperative sensitivity, marginal staining, recurrent caries and microleakage. Application of low-shrinkage composites is a key strategy to reduce polymerization shrinkage stress [6,7]. Two main approaches to reduce polymerization shrinkage include using different types of resin monomers and reduction of reactive sites per volume unit (by increasing filler loading and/or increasing molecular weight per reactive group) [8]. Kalore (GC Corporation, Tokyo, Japan) is a lowshrinkage nano-composite with 82% filler loading Patients with poor oral hygiene, parafunctional habits, serious health problems, a known allergy to the substances used in this study, history of existing tooth hyper-sensitivity, pregnancy or breast-feeding, chronic use of anti-inflammatory, analgesic, and psychotropic drugs, xerostomia and periodontal diseases were excluded. Bitewing radiographs of the teeth to be restored were taken preoperatively. Clinical procedure: The teeth to be restored were cleaned with a slurry of pumice, rinsed with water and then air dried. The conservative cavity was prepared using a diamond bur (Tizcavan, Tehran, Iran) with a high-speed hand-piece. New burs were used after every five preparations. All carious tooth structures were removed using a low-speed hand-piece and a steel round bur. In very deep cavities (less than approximately 0.5mm of dentin) the dentin was covered with calcium hydroxide (Dycal, Dentsply, Caulk, Milford, USA) and resin modified glass-ionomer cement (GC Corporation, Tokyo, Japan); whereas, in deep cavities (the residual dentin thickness was 0.5-1.5mm) dentin was covered solely with resin-modified glass-ionomer cement. www.jdt.tums.ac.ir March 2017; Vol.14, No.2 . The teeth to be restored were randomly filled with a packable composite (P60), or a lowshrinkage methacrylate-based composite (Kalore GC). The resin composites were placed in straight increments not exceeding 2mm in thickness, and adapted with a flat-faced condenser and then each increment was light cured for 40 seconds. The materials used in this study with their chemical compositions are presented in Table 2.
All restorations were finished with fine grit finishing diamond burs (Diatech; Dental AG, Heerbrug, Switzerland) and polished with polishing rubber points (Edenta Composite Polishing Kit; AU, St. Gallen, Switzerland) and Enhance (Dentsply Latin America, Petropolis, RI, Brazil). Clinical evaluation: All restorations were evaluated one week after placement, six months and one year. Two calibrated clinicians, other than the operator, evaluated the restorations blindly at each recall using the modified United States Public Health Service (USPHS) criteria (Table 3). The two examiners, patients and analyzer were unaware of the type of composite used (triple-blind design). The evaluated parameters were color match, marginal adaptation, retention, anatomical form, sensitivity, surface roughness, marginal staining and secondary caries. For each criterion, Alpha score represented ideal clinical situation, Bravo indicated clinically acceptable, and Charlie indicated clinically unacceptable situation.
When disagreements occurred during the evaluation of restorations, a consensus was reached between the two examiners. Data were analyzed using SPSS version 18.0 (SPSS Inc., IL, USA) and the Friedman and conditional (matched) logistic regression tests at P<0.05 level of significance.

RESULTS
Overall in 25 patients, fifty restorations were placed and all restorations were evaluated at six months and one-year follow-up. The results are summarized in Table 4. The results of the present study indicated that all of the restorations at baseline exhibited Alpha score for all criteria (except for postoperative hypersensitivity and color match). Four patients experienced postoperative hypersensitivity (slight discomfort associated with cold beverage and mastication) after restoration placement (two P60 and two Kalore restorations), which disappeared gradually after three months.  A  25  25  25  25  25  25  B - A  25  25  25  25  25  24  B - A  25  25  25  25  25  25  B  ------C  ------No statistically significant differences were found between the two composites at six months and one year of service (P>0.05). Moreover, when each composite was evaluated independently at baseline and after one year, no statistically significant differences were found (P>0.05) except in P60 group for marginal adaptation (P<0.05). No secondary caries, lack of retention and loss of anatomical form were found in any of the studied materials after one year (100% Alpha). At the six-month recall, 6% of restorations (2% Kalore and 4% P60) showed Bravo score for marginal adaptation and 2% (P60) for surface roughness. After one year of follow-up, following restorations were recorded as Bravo: 8% surface roughness (2% Kalore and 6% P60), 12% marginal adaptation (4% Kalore and 8% P60) and 2% marginal staining (P60).

DISCUSSION
The result of this study showed that when each composite resin was evaluated independently at baseline and after one year, no statistically significant differences were found except in P60 group that showed significant difference in marginal adaptation. The postoperative hypersensitivity in posterior composite restorations is reported as a common problem in operative dentistry. Postoperative hypersensitivity may be due to several factors such as etching of dentin, bacterial microleakage, cuspal flexure, cavity depth, technique of restoration placement, over-drying of dentin, occlusal interferences, incorrect adhesive procedure, cavity size and trauma caused by cavity preparation [19,20]. Variations in the occurrence of postoperative sensitivity have also been reported among different clinicians with respect to their techniques and experiences [19]. Our results indicated that marginal adaption in 12% of restorations (8% Filtek P60 and 4% Kalore) rated Bravo after one year of follow-up and no significant difference was found between the two experimented composites. The marginal adaptation of composite restoration is influenced by several factors such as type of dentin adhesive, the polymerization shrinkage of composites, restoration technique and accuracy of the finishing procedure [21,22]. Some studies have reported that many of marginal defects result from the fracture of thin areas of composite flash or excessive adhesive and these external flashes can be removed by better finishing and polishing [12,22]. The results of our study exhibited that after one year, 8% of restorations (2% Kalore and 6% P60) showed Bravo score for surface roughness. Consequently, Kalore restorations showed relatively better performance in comparison with Filtek P60, although no significant difference was recorded between the two types of composites. However, marginal adaption of P60 was significantly worse (but clinically acceptable) at one year than at baseline. The surface roughness of composite restorations depends on some internal factors such as filler (type, size, shape, hardness and loading), type of resin matrix, ultimate degree of conversion and quality of bond between filler particles and resin matrix [23]. The external factors include the techniques of finishing and polishing, type of finishing and polishing used and light curing method [24]. In the current study, Filtek P60 expressed relatively higher surface roughness; it could be related to the deficiency of bonding between the matrix and the fillers resulted from nonsilanization of the latter. This may cause protrusion of some filler particles as the weak resin matrix is lost during the procedure of finishing and polishing, and as the result, the surface becomes rough [25]. Moreover, Filtek P60 contains high molecular weight Bis-EMA and UDMA resulting in a slightly softer matrix because of forming fewer double bonds and thereby increasing the surface roughness [24]. On the other hand, some experiments have indicated that microhybrid composites in comparison with nano-filled composites show smaller volume and less homogenous distribution of inorganic fillers. A surface formed by nanoparticles usually shows less particle loss (and thus increased surface roughness), as compared with microhybrid composites [26]. In the current study, no failure was detected among restorations during the one-year period. Collin et al, [27] in 1998 reported that recurrent caries and bulk fracture are the main factors responsible for posterior composite failure and in their study, secondary caries and bulk fracture were not observed. Clinical success of posterior composite restorations depends on several factors including patient caries risk, oral hygiene and age, clinical factors (size, location and type of restoration), socioeconomic factors and operator-related factors (knowledge, skills, quality of work and technique) [28]. Although one or two-year studies may provide useful information on the performance of newly introduced composites and their catastrophic failure, precise assessment of longevity of